Nano's Blog

The shift and unshift operators perform the corresponding actions on the “start” of the array (or the “left” side of an array, or the portion with the lowest subscripts).
push and pop operators do things to the “end” of an array(or the “right” sidce of an array).

The foreach control involved to the sample code below.

#!C:\Perl\bin\perl.exe
use strict;
use warnings;

my @array = qw# dino fred barney #;
# print @array;
foreach my $array (@array) {
$array = "\t$array"; # put a tab in front of each element of @rocks
$array.= "\n"; # put a newline on the end of each
print $array;
}

my $m = shift(@array); # $m gets "dino", @array now has ("fred", "barney")
my $n = shift @array; # $n gets "fred", @array now has ("barney")
shift @array; # @array is now empty
# print a empty array
foreach my $array (@array) {
$array = "\t$array"; # put a tab in front of each element of @rocks
$array.= "\n"; # put a newline on the end of each
print $array; # nothing printed
}

my $o = shift @array; # $o gets undef, @array is still empty
unshift(@array, 5); # @array now has the one-element list (5)
unshift @array, 4; # @array now has (4, 5)
my @others = 1..3;
unshift @array, @others; # @array now has (1, 2, 3, 4, 5)
foreach my $array (@array) {
$array = "\t$array"; # put a tab in front of each element of @rocks
$array.= "\n"; # put a newline on the end of each
print $array;
}

Output:

D:\learning\perl>hi.pl
        dino
        fred
        barney
        1
        2
        3
        4
        5

D:\learning\perl>

- from Learning Perl 5E

If you wish to iterate over (that is, examine every element of) an entire hash, one of the usual ways is to use the each function, which returns a key-value pair as two-element list.

#!C:\Perl\bin\perl.exe
use strict;
use warnings;

# declare a new hash
my %some_hash;

%some_hash = ("foo", 35, "bar", 12.4, 2.5, "hello",
      "wilma", 1.72e30, "betty", "bye");

my $key;
my $value;

while ( ($key, $value) = each %some_hash)
{
  print "$key => $value\n";
}

print "\n";
foreach $key (sort keys %some_hash) {
  $value = $some_hash{$key};
  print "$key => $value\n";
  # Or, we could have avoided the extra $value variable:
  #  print "$key => $hash{$key}\n";
}

Output

betty => bye
bar => 12.4
wilma => 1.72e+030
foo => 35
2.5 => hello

2.5 => hello
bar => 12.4
betty => bye
foo => 35
wilma => 1.72e+030

-from Learning Perl 5E

The order is jumbled because Perl keeps the key-value pairs in an order that’s conven-
ient for Perl so that it can look up any item quickly.

Elements of a hash are printed out in their internal order, which can not be relied upon and will change as elements are added and removed. If you need all of the elements of a hash in some sort of order, sort the keys, and use that list to index the hash.

If you are looking for a structure that holds its elements in order, either use an array, or use one of the ordered hash’s on CPAN.

the only ordering you can rely upon from a list context hash expansion is that key => value pairs will be together.

use the keys function to get a list of the keys. They will most likely be in the same order as they’re stored in the hash itself, although there’s no guarantee. If the order matters, then you either shouldn’t be using a hash in the first place, or you should use the keys function to get a list, put the items in the desired order, and then loop through that list, accessing each corresponding hash item.

#!C:\Perl\bin\perl.exe
use strict;
use warnings;

# declare a new hash
my %some_hash;

%some_hash = ("foo", 35, "bar", 12.4, 2.5, "hello",
      "wilma", 1.72e30, "betty", "bye");

my @any_array;
@any_array = %some_hash;

print %some_hash;
print "\n";
print "\n";
print @any_array;
print "\n";
print "\n";

my @keys;
@keys = keys %some_hash;
for my $k (sort @keys)
{
    print $k, $some_hash{$k};
}
print "\n";
print "\n";
print map {$_, $some_hash{$_}} sort keys %some_hash;

Output

D:\learning\perl>test.pl
bettybyebar12.4wilma1.72e+030foo352.5hello

bettybyebar12.4wilma1.72e+030foo352.5hello

2.5hellobar12.4bettybyefoo35wilma1.72e+030

2.5hellobar12.4bettybyefoo35wilma1.72e+030
D:\learning\perl>

-from StackOverflow.com